How to use Clone Brush in Image Editor?
Clone Brush in the Image Editor (2D) doesn't work at all the same -you cannot use the cursor as a reference - you instead use a transfer or clone image that is the same size as the target image, and then move it about with rmb to position it and then paint through to clone. This is a VERY limited functionality, and there is no rotation of the image. For this reason, I prefer to use Stencil Brush Texture instead since we can rotate/scale/move and even set the UV to be upside down/repeating etc. Honestly, that is why it is sometimes better to just work on a 2d image plane in the 3d viewport than to waste time with the image editor for anything other than fast paint dabs or Fill tool control.